1. Field of the Invention
The present invention relates to a conveyor unit, and more specifically to a conveyor unit capable of controlling conveyance without using a sensor for detecting articles. The present invention also relates to a zone controlled conveyor system incorporating the conveyor unit.
2. Description of the Related Art
A zone controlled conveyor system for controlling conveyance having a conveyor line consisting of a plurality of control zones (zones for control), each zone having a conveyor unit for controlling conveyance of articles has been developed.
A patent document 1 specified below discloses such a conveyor system.
The conventional conveyor system that is disclosed in the patent document 1 includes a conveyor unit provided with a zone controller for controlling a driving roller in each control zone. The controllers of adjacent control zones, mutually connected, drive and halt operations of the driving roller depending on presence signals of each article of its home control zone (i.e., the zone corresponding to the zone controller in question), taking into account signals such as presence signals transmitted from upstream and downstream control zones. More specifically, according to the conventional conveyor system disclosed in the patent document 1, such cooperative control by zone controllers achieves the “zero-pressure accumulating control” that enables articles to be conveyed toward a conveying direction, avoiding collision therebetween.
The conventional system requires a sensor for detecting an article provided at each control zone, resulting in being obliged to attach components such as the sensor or a cable for connecting the sensor to the controller at each conveyor unit. That increases the number of components, causing increased cost and troublesome construction of the system.
In order to solve the problem described above, a conveyor unit that conveys articles dispensing with a sensor has been developed. Patent documents 2 and 3 specified below disclose such a conveyor unit that eliminates a sensor.
The conveyor unit disclosed in the patent document 2 generates pulses depending on rotational speed of a motor or a driving roller and compares the generated pulses with predetermined reference pulses, thereby detecting whether an article is conveyed into the home control zone or out of the zone.
The conveyor unit disclosed in the patent document 3 makes a normal rotation and a reverse rotation of a motor during conveyance of an article and detects a difference of rotational speed of the motor at this time by a difference of the number of pulses generated from the motor, thereby determining conveyance of the article into the home control zone or out of the zone.
These conventional units disclosed in the patent documents 2 and 3 dispense with a sensor to detect whether an article is conveyed into the home control zone or out of the zone, thereby enabling reduction of cost and the trouble of construction of the system.
Patent Document 1: JP 11-199030 A
Patent Document 2: JP 2000-335730 A
Patent Document 3: US 2003-0209410 A1
However, the conveyor unit disclosed in the patent document 2 employs a magnetic-pole detector such as a Hall element detecting a rotational position of a rotor to generate pulses depending on a rotation of a motor or a driving roller. A status of conveyance of an article cannot be determined until the magnetic-pole detecting element detects a magnetic pole reversal, until the motor or the driving roller reaches a reference rotation number, or until a reference time has passed, causing a failure that the status of conveyance cannot be determined precisely and in a short period of time.
The conveyor unit disclosed in the patent document 3 cannot determine a status of conveyance of an article unless the motor is normally and reversely rotated, requiring time to determine the status. Thus, some improvement has been desired.
Therefore, the inventors of the present invention experimentally created a conveyor unit adapted to detect an article conveyed thereon by use of an electromotive force generated by an externally forced rotation of a motor.
FIG. 16 is a block diagram showing an operating principle of the conveyor unit 100 experimentally created by the inventors. The conveyor unit 100 includes a drive control section 101, a motor driving section 102, and a motor 103.
When an article is conveyed on the conveyor unit 100, the drive control section 101 generates a driving signal in response to a rotational position sensing signal outputted from Hall elements 104 depending on a rotational position of a rotor (not shown), whereupon the motor driving section 102 sequentially applies current to each of armature coils 103a to 103c to drive rotation of the motor 103 in response to the driving signal generated by the drive control section 101.
When the motor 103 is not driven, an external rotational force that is caused by conveyance of the article and acts on the motor 103 induces an electromotive force in the armature coils 103a to 103c. As shown in FIG. 16, the electromotive force induced in the armature coils 103a and 103b is derived as a detection signal, thereby determining an externally forced rotation of the motor 103; that is, whether the article is conveyed into an area powered by the motor 103 or onto the conveyor unit 100.
Although it is possible, in principle, that the conveyor unit 100 shown in FIG. 16 detects conveyance of the article onto the conveyor unit, it was in a critical problem in actually designing the device.
FIG. 17 is a graph showing the rotational position sensing signal and the electromotive force in the conveyor unit 100 shown in FIG. 16 with their time axes aligned, the sensing signal being outputted from the Hall elements 104 in an externally forced rotation of the rotor (not shown) of the motor 103, the electromotive force being generated by the armatures.
Referring to FIG. 17, at the time of non-drive of the motor 103, an externally forced rotation of the motor 103 induces an electromotive force with an increased peak value (crest value) in the armature coils 103a and 103c in proportion as shortening of pulse width of the rotational position sensing signal (i.e., as increasing of the speed of the rotation).
However, in a place where the conveyor unit 100 is installed, fluctuations of magnetic field generated by adjacent conveyor units 100 and ground potential are liable to occur, causing the armature coils 103a to 103c to suffer from many noise components.
Thus, during slow speed of the externally forced rotation of the motor 103, the electromotive force induced in the armature coils 103a and 103c may be undetectable being lost among the noise components. That results in such a problem that threshold value in the electromotive force level for determining the rotation of the motor 103 must be set high to avoid a false detection because of the noise.
More specifically, the conveyor unit 100 shown in FIG. 16 has an excellent structure utilizing the electromotive force of the motor 103, nevertheless it may cause contradictory problems such that low setting of the threshold value of detection of the electromotive force leads to a false detection because of the noise, whereas high setting of the threshold value requires substantial time to detect the rotation of the motor 103, and thus it was difficult to design and to be practicable.
Taking into account the above-mentioned problems, the present invention proposed herein is to provide a conveyor unit adapted to take advantage of a function of power generation of a motor to stably detect conveyance of articles thereon, and further adapted to be readily designed. The present invention proposed herein is also to provide a zone controlled conveyor system incorporating the conveyor unit.